We need to make a bunch of behavior changes before we can drop prefixes on transforms. From memory:
1. Require units on perpective (function and property)
2. Implement transform function matching for animation according to the spec
3. Implement the 3D rendering context stuff properly (layer intersection)
4. Change background-attachment:fixed behavior.
5. Probably more.
We'd want the prefixed properties to retain the current behavior.

(In reply to comment #3)
> We need to make a bunch of behavior changes before we can drop prefixes on transforms. From memory:
> 1. Require units on perpective (function and property)
> 2. Implement transform function matching for animation according to the spec
> 3. Implement the 3D rendering context stuff properly (layer intersection)
> 4. Change background-attachment:fixed behavior.
> 5. Probably more.
Yeah, I figured I should have made this into three different bugs for the three areas before even asking... I'm mostly looking at Animation at this point, and so far haven't noticed anything crucial to be missing. I guess I should write to webkit-dev before wasting too much effort though.
> We'd want the prefixed properties to retain the current behavior.
Good to know, thanks.

According to http://caniuse.com/#feat=css-animation the upcoming version of Chrome (39) will unprefix CSS animations. Opera will follow suit. This means the last browser not support unprefixed CSS animations will be webkit.
Android Browser and Chrome for Android will follow suit 2-3 months later (they are 2 versions behind the desktop version).